This invention relates to fluid engines far deriving work from fluid cycles and more particularly to improvements in Stirling cycle techniques.
There is now a well recognized need for fuel burning engines whose emissions are acceptably free of pollutants. Given fuels that initially have negligible amounts of lead and sulfur, the primary objectionable emissions from modern internal combustion engines are unburned hydrocarbons, carbon monoxide and oxides of nitrogen. Numerous control systems have been devised for conventional engines, based on mixture control, recirculation techniques, emission purification and various combinations of these. While improvements are constantly being made, some mutually contradictory factors prevent rapid achievement of substantially lowered levels for emitted pollutants.
Briefly, these factors are the concurrent need for reasonable power output, pollution control under both transient and steady state conditions, efficient operation and avoidance of unreasonable cost and upkeep penalty. For example, superimposition of controls on existing engines adversely affects both cost and performance. Basic redesign of the common Otto cycle engine to satisfy all requirements does not appear feasible, because these engines operate on an intermittent cycle that is not compatible with efficient burning under widely varied conditions. The Wankel engine, which is in increasing use, inherently does not burn any more cleanly. Recirculation and afterburner systems have been utilized in attempts to reduce levels of pollutants in emissions from both types of engines, and current thinking in the automotive industry appears to regard these corrective systems as necessary if increasingly stringent federal pollution controls are to be met.
Consideration has consequently been given to engines that have inherently superior characteristics in terms of pollution control. One such engine operates on the Stirling cycle, first suggested in the early 19th Century and since used in a variety of applications. The most prevalent current use is in cryogenic systems, in which the work output of the engine is derived in terms of a refrigeration capacity. The Stirling cycle, which need not be reviewed here in detail, utilizes contraction and expansion of heated and cooled intercommunicating gas volumes in timed relation to the extraction of work energy. Engines utilizing this cycle have the additional virtue of generating very little noise. Heretofore, Stirling cycle engines have generally employed external combustion, and although internal combustion was suggested in an early patent to Hirsch, No. 155,087, no present examples of such a Stirling engine are known. External combustion necessitates certain refined techniques to achieve desired efficiency, and these have militated against the use of such engines in vehicles or other applications requiring substantial power generation. External heating of a chamber inherently requires substantial start-up time, and additionally necessitates use of significant amounts of costly and limited high temperature metal (i.e., nickel). Improved efficiency is attained by use of working fluids having low density and high thermal conductivity; helium or hydrogen have been most often used. No convenient mass distribution means is currently available for vehicular users of these gases, which must also be separately stored in the vehicle and thus introduce additional cost factors. In addition, their high thermal conductivities combined with the characteristically relatively constant elevated temperatures of Stirling engines require use of substantial thermal barriers to prevent heat wastage. While the weight and start-up time of the engines can be reduced, there is inevitably a significant penalty in terms of cost and complexity.
In addition there are additional practical considerations when Stirling engines are used in vehicular applications. The work output must be rapidly controllable in order to accelerate and decelerate the vehicle, a factor that requires further special modification of existing systems. Weight, size, power output and work efficiency must reasonably approach or approximate the performance of existing internal combustion systems.
In a Stirling cycle engine that is required to limit oxides of nitrogen, full advantage of theoretical work output cannot be gained because hot gas temperature must be reduced. Particularly if higher hot gas temperatures can be used while the other disadvantages of current Stirling engines are overcome, it is evident that many new applications for the engines exist. For example, the engines may be used for stationary power generating units, or they may be used to extract a substantial proportion of thermal energy available in the high temperature effluent from existing power generating systems.
The state of the art in Stirling engines is evidenced in; addition to the patent to Hirsch, by representative patents such as the following:
Cowans -- 3,379,026 PA1 Van Nuckeren -- 2,484,392 PA1 Jonkers -- 2,657,552 PA1 Jonkers -- 2,657,553